Add a tagged output for app's exportPackage

This allows the android stubs to depend on framework-res' exportPackage
without special-casing inside the build system.

Bug: 161214753
Test: depending on this output in a followup CL
Change-Id: I8c5d17540d2624974983d73e56ba17898505dba8

java/app.go

@@ -965,6 +965,8 @@ func (a *AndroidApp) OutputFiles(tag string) (android.Paths, error) {
 	switch tag {
 	case ".aapt.srcjar":
 		return []android.Path{a.aaptSrcJar}, nil
+	case ".export-package.apk":
+		return []android.Path{a.exportPackage}, nil
 	}
 	return a.Library.OutputFiles(tag)
 }